a. Field of the Invention
This invention relates to a steer-by-wire system, and more particularly it relates to a fail-safe mechanism for steer-by-wire systems, so designed that when a defective condition develops in a steer-by-wire system, a shaft connected directly to the steering wheel is connected to a shaft connected to the steering gear, so as to make direct steering by the steering wheel possible.
b. Related Technology
Steer-by-wire systems replace a system in which the steering wheel and the steering gear of an automobile are not mechanically connected, and are adapted to control the steer angle by rotating a motor through electric signals. Since steer-by-wire systems, which dispense with steering shaft and the like for connecting the steering wheel and steering gear, increase the degree of freedom of car interior layout and allow automatic control of road wheel steer angle according to the vehicle speed; thus, steer-by-wire systems are expected to improve safety travel and motion performance of vehicles.
FIG. 14 schematically illustrates a steer-by-wire system 1. A steering wheel 2 and a steering gear 3 are not mechanically connected, and instead the steer angle of the steering gear 3 is operated through a steer actuator 5 controlled by a controller 6 according to the steer angle of the steering wheel 2, and a reaction simulator 4 imparts a steer reaction controlled by the controller 6 to the steering wheel 2.
In the steer-by-wire system in which the steering wheel 2 and the steering gear 3 are not mechanically connected in this manner, if a defective condition should develop in the steer actuator 5 or reaction simulator 4, a defective condition would develop in the control of the vehicle by steering. Thus, Japanese Patent Application Laid-Open under No. 2001-301639 discloses a fail-safe mechanism therefor, in which the steering wheel and the steering gear are connected by a solenoid clutch to allow direct steering by the steering wheel.
In the steer-by-wire system disclosed in Japanese Patent Application Laid-Open under No. 2001-301639, since a solenoid clutch is used, it follows that when supply of electric power is cut off as by a fault in the electric system, the solenoid clutch cannot be switched, thus making it impossible to connect the steering wheel and the steering gear. Further, since the solenoid clutch is a friction transmission type clutch, more or less electric power supply capability is required and moreover it is difficult to make the whole system compact. Further, a friction transmission type clutch, such as a solenoid clutch, has the demerit of drag torque during dragging being high.